I think what a lot of people here misunderstand is what "Overpowered" means. Everything has a counter. Even a mewtwo with +6 SPatk and +6 speed can still be killed if you know that you'll be facing that. Still, Mewtwo is overpowered, or "Uber". That is because in a normal game, it's hard to counter Mewtwo without using Ubers. Even if your prediction is flawless, if you don't have the right team, you lose to Mewtwo. Of course, this can be said for anything: However, you'll be forced to take some things especially for Mewtwo, that have no use besides countering Mewtwo. Now you're essentially forced to use an extra moveslot, which weakens the overall strength of your team. That's what makes Mewtwo Uber.
Now, let's take a look at Mew. His stats are 100 all around, which isn't very interesting. Yet, Mew is Uber. Why? Versatility. Let's say you see a Mew in a match. A lot of Mews are batton passers, so you switch to your Phazer (let's take Swampert as an example). However, instead of statting up, you eat a Spec'd Grass Knot. Next time you see a Mew, you don't know whether it's going to Pass, or be a Specs. So you switch in Blissey, who's packing Sing, and... whoops. Mew is a Choicebander this time.
Mew has so much variety, that there is no set counter. Until you have lost one of your Pokemon (or, you use protect on the first turn it can attack, but by then it could've gotten a swords dance in), you have no way to know what kind of Mew it is, and how to counter it. This means Mew alone can demolish an entire team: Before you can react to it, Mew will already have passed some stats to a sweeper, and you'll have a lot of trouble even staying alive, let alone make a comeback. That is why Mew is Uber.
Now, let's take a look at why Garchomp should be Uber.
1) It is very strong, and you need a set counter to it
2) There are multiple builds (suicide sweeper, chainchomp, SD/sub), that are vastly different, so you have no way of knowing how to get rid of the monster.
Now that point 2 has popped up, Garchomp can become a problem. Before, there was only 1 kind of build, and even though Garchomp is strong, it's not too strong. It can almost always be countered by one specific Pokemon, so it's not a really big problem. But now that there are multiple builds, you have to take specific counters to a Pokemon that can easily rape a whole team. To find out what kind of Garchomp it is, so you can counter it, you'll already be crippled: Either the Chomp stats up, or one of your pokemon is (near) dead. Only then can you switch your counter in, if you have a Pokemon that can take care of this Garchomp build.
And then you miss your attack.
Because of Sand veil, it's possible to lose your only way of saving your team because of something you can't control. Yes, it's also possible to lose because your Stone edge or your Hypnosis missed. However, these are things that you can control: You can take Rock Slide instead of SE, or dump Hypnosis. However, there is nothing you can do about Sand Veil, but change the weather.
Weather-changing, however, is inefficient. Generally, non-SS weather teams are bad (with the exception of the recently discovered awesomeness of hail teams). Sunlight sucks, as there's no ability to bring it up in OU and you won't have time to use the move when there's a Garchomp on the loose. Same with Rain. Hail is the only one that's able to counter Garchomp. However, the best way to get it in is with Abomasnow's ability. If Garchomp comes in while SS is up, that means you'll have to sacrifice Abomasnow just to get rid of the SS, heavily crippling your team.
However, if you don't use a hail-team, as it is quite limiting, you're F-ed when Sand Veil is up. There's nothing you can do about missing because of Sand Veil, which sucks.